1. Field of the Invention
The present invention relates generally to the field of biomedical analysis, and particularly to an apparatus and method for non-invasively determining the cardiac output in a living subject using impedance cardiography.
2. Description of Related Technology
Noninvasive estimates of cardiac output (CO) can be obtained using impedance cardiography. Strictly speaking, impedance cardiography, also known as thoracic bioimpedance or impedance plethysmography, is used to measure the stroke volume of the heart. As shown in Eq. (1), when the stroke volume is multiplied by heart rate, cardiac output is obtained.
CO=stroke volumexc3x97heart rate.xe2x80x83xe2x80x83(1)
The heart rate is obtained from an electrocardiogram. The basic method of correlating thoracic, or chest cavity, impedance, ZT(t), with stroke volume was developed by Kubicek, et al. at the University of Minnesota for use by NASA. See, e.g., U.S. Reissue Patent No. 30,101 entitled xe2x80x9cImpedance plethysmographxe2x80x9d issued Sep. 25, 1979, which is incorporated herein by reference in its entirety. The method generally comprises modeling the thoracic impedance ZT(t) as a constant impedance, Zo, and time-varying impedance, xcex94Z (t), as illustrated schematically in FIG. 1. The time-varying impedance is measured by way of an impedance waveform derived from electrodes placed on various locations of the subject""s thorax; changes in the impedance over time can then be related to the change in fluidic volume (i.e., stroke volume), and ultimately cardiac output via Eqn. (1) above.
Despite their general utility, prior art impedance cardiography techniques such as those developed by Kubicek, et al. have suffered from certain disabilities. First, the distance (and orientation) between the terminals of the electrodes of the cardiography device which are placed on the skin of the subject is highly variable; this variability introduces error into the impedance measurements. Specifically, under the prior art approaches, individual electrodes 200 such as that shown in FIGS. 2a and 2b, which typically include a button xe2x80x9csnapxe2x80x9d type connector 202, compliant substrate 204, and gel electrolyte 206 are affixed to the skin of the subject at locations determined by the clinician. Since there is no direct physical coupling between the individual electrodes, their placement is somewhat arbitrary, both with respect to the subject and with respect to each other. Hence, two measurements of the same subject by the same clinician may produce different results, dependent at least in part on the clinician""s choice of placement location for the electrodes. It has further been shown that with respect to impedance cardiography measurements, certain values of electrode spacing yield better results than other values.
Additionally, as the subject moves, contorts, and/or respirates during the measurement, the relative orientation and position of the individual electrodes may vary significantly. Electrodes utilizing a weak adhesive may also be displaced laterally to a different location on the skin through subject movement, tension on the electrical leads connected to the electrodes, or even incidental contact. This so-called xe2x80x9cmotion artifactxe2x80x9d can also reflect itself as reduced accuracy of the cardiac output measurements obtained using the impedance cardiography device.
A second disability associated with prior art impedance cardiography techniques relates to the detection of a degraded electrical connection or loss of electrical continuity between the terminals of the electrode and the electrical leads used to connect thereto. Specifically, as the subject moves or sweats during the measurement, the electrolyte of the electrode may lose contact with the skin, and/or the electrical leads may become partially or completely disconnected from the terminals of the electrode. These conditions result at best in a degraded signal, and at worst in a measurement which is not representative of the actual physiological condition of the subject.
Another significant consideration in the use of electrodes as part of impedance cardiographic measurements is the downward or normal pressure applied to the subject in applying the electrode to the skin, and connecting the electrical leads to the electrode. It is desirable to minimize the amount of pressure needed to securely affix the electrode to the subject""s skin (as well as engage the electrical lead to the electrode), especially in subjects whose skin has been compromised by way of surgery or other injury, since significant pressure can result in pain, and reopening of wounds.
Based on the foregoing, there is a need for an improved apparatus and method for measuring cardiac output in a living subject. Such improved apparatus and method ideally would allow the clinician to repeatedly and consistently place the electrodes at the optimal locations. Additionally, such an improved apparatus and method would also permit the detection of degraded electrical continuity between the electrode terminal and skin, or the electrode terminal and electrical leads of the measurement system, and be adapted to minimize the normal pressure on the subject""s tissue when applying the electrodes and electrical leads.
The present invention satisfies the aforementioned needs by providing an improved method and apparatus for measuring the cardiac output of a living subject.
In a first aspect of the invention, an improved apparatus for determining the cardiac output of a living subject is disclosed. The apparatus generally comprises: a plurality of electrode assemblies having a plurality of terminals, at least two of the plurality of terminals being spaced from one another by a predetermined distance; a current source capable of generating a substantially constant current; a plurality of electrical leads connecting the current source with individual ones of the terminals of the electrode assemblies, a circuit for measuring the difference in voltage at the terminals resulting from the flow of current through the subject and the terminals; and a circuit for measuring ECG potentials from at least one of the electrode assemblies. In one exemplary embodiment, the subject is a human being. Cardiac stroke volume is measured by applying a constant current to the stimulation electrodes, measuring the resulting voltage differential, and determining the stroke volume from the measured voltage and a predetermined relationship describing intra-thoracic impedance.
In a second aspect of the invention, an improved cardiac electrode apparatus is disclosed. The apparatus generally comprises: a substrate having a plurality of apertures formed therein, at least two of the apertures being formed a predetermined distance apart; a plurality of terminals disposed within respective ones of the apertures, at least a portion of each of the terminals being capable of conducting an electrical current; and at least one gel element being adapted to transfer electrical current between the skin of the subject and at least one of the plurality of terminals. In one exemplary embodiment, the electrode apparatus comprises a pair of xe2x80x9csnapxe2x80x9d terminals disposed a predetermined distance apart within the substrate and which can be readily and positively connected to using jaw-type connectors. The electrode apparatus is adapted to mate uniformly with the skin of the subject, and maintain the desired contact with the skin as well as the predetermined spacing between the electrode terminals.
In a third aspect of the invention, an improved method of measuring the cardiac output of a living subject is disclosed. The method generally comprises: providing a plurality of electrode arrays each having a plurality of terminals, at least two of the terminals being spaced a predetermined distance apart; positioning the electrode arrays at respective locations in relation to the thoracic cavity of the subject; generating an electrical current, the current passing from a first electrode of at least one of the electrode arrays through the subject and to a second electrode of at least one of the arrays; measuring the voltage at the second electrode; determining stroke volume from the measured voltage; and determining cardiac output based at least in part on the stroke volume. In one exemplary embodiment, four electrode pairs are utilized, each having predetermined terminal spacing. The electrode pairs are placed at various locations above and below the thoracic cavity of the subject, on both sides of the cavity. Both differential voltage and cardiac rate are measured via the electrode pairs.
In a fourth aspect of the invention, a method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system is disclosed. The method generally comprises: providing a plurality of electrically conductive terminals; disposing the terminals in relation to the thoracic cavity of a subject; generating a current between a first of the terminals and a second of the terminals, the current passing through at least a portion of the thoracic cavity; obtaining an impedance waveform from the second terminal; and comparing the impedance waveform to a similar waveform obtained from another of the terminals; wherein the difference between the waveforms is used to evaluate the electrical continuity of the terminals.